Lithographic analysis of multipass gray writing strategy for electron-beam pattern generation

Gray-level printing is an efficient strategy to create small-address patterns on photomasks, This work provides a technical description of the multipass gray (MPG) raster-scan writing technique as implemented on the MEBES(R) 4500S and MEBES 5000 electron-beam pattern generation tools. The differences between single-pass printing (SPP) and MPG are reviewed. The factors that allow increases in throughput and dose with MPG are explained. Aerial image simulations of edge placement and corner rounding verify the MPG model. Multipass writing with offset scan voting (OSV), which reduces random and systematic errors, is explained. Because MPG is a gray-level printing technique, the dose distribution across feature edges is necessarily broader than that derived from SPP writing. Simulations and experimental results indicate that, using ZEP 7000 resist and dry etch, edges can he placed without loss of accuracy, despite the width of this "gray" profile. The spot size necessary to obtain optimal critical dimension (CD) quality is also determined by simulation and empirically. The lithographic quality of MPG writing/processing is confirmed by composite metrology tests that sample the whole quality area of the mask. We conclude that MPG is a viable technique for writing advanced masks.